The present invention relates to the general field of gas turbine aeroengines. It relates more particularly to controlling the rate at which fuel is injected into a turbine engine.
In an aeroengine, it is known to determine a fuel-flow-rate setpoint for application to a fuel metering unit of a turbine engine as a function of the difference between the speed of the engine and a setpoint speed that depends on the position of a control lever operable by the pilot. For this purpose, a regulation loop is implemented by an electronic control unit of the engine.
In order to protect the engine against the risk of pumping during speed changes, the regulation loop may include a stop, referred to as the C/P stop, which stop limits the maximum rate at which fuel can be injected. Under such conditions, the time required to accelerate or decelerate the engine depends directly on the pumping margin of the engine.
Thus, an old or worn engine will present an acceleration or deceleration time that is longer than a new engine. Furthermore, utilization conditions (atmospheric conditions, flight envelope, power takeoff, . . . ) have an influence on the acceleration or deceleration time. This leads to a lack of reproducibility in the time required to accelerate or decelerate a given engine, and also to a lack of conformity between a plurality of engines of the same type, and this can lead to thrust asymmetry when accelerating.
It is also known to protect an engine against the risk of pumping during speed transients by using a regulation loop based on complying with an acceleration setpoint. Documents U.S. Pat. No. 4,543,782 and US 2003/0094000 describe systems of that type. There likewise, it is not possible to ensure acceleration or deceleration times that are reproducible. Following a rate-of-change for the speed does not make it possible to catch up for any delay acquired at the beginning of a transient.
Document EP 0 324 633 describes a method of controlling a turbine engine in which a fuel-flow-rate setpoint is determined in particular by making use of a “GOVERNOR” loop based on a difference between a speed setpoint NLdat that depends on the position of the control lever and the actual speed NL, and of a loop “DECEL LOOP” based on a difference between a speed that follows a determined trajectory by integrating a slope setpoint NHdet(−) and the speed NH. A “highest win” type gate selects the value supplied by one or other of the “GOVERNOR” and “DECEL LOOP” loops.
In similar manner, in EP 0 092 426, one of the values supplied by the regulation loops is selected by a “lowest win” type gate.
Nevertheless, selecting a flow rate setpoint by means of a “lowest win” or a “highest win” type gate presents drawbacks with a speed setpoint that is based on following a trajectory. During a fast transient, the inertia that needs to be overcome in order to accelerate the turbine engine make it necessary, at a given speed, to inject fuel at a rate that is much greater than the rate needed for stabilizing the engine at the same speed. Under such circumstances, “lowest win” type selection logic leads to premature selection of the main loop, thereby truncating acceleration. This phenomenon leads to the acceleration time not being complied with and also to non-reproducibility between accelerations.